In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This includes the width and spacing of interconnecting lines, gate conductors, trenches, vias and various other devices and features that are formed with the aid of lithography. Since numerous devices, interconnecting lines and other features are typically present on a semiconductor wafer, the trend toward higher device densities is a notable concern.
The requirement of small features (and close spacing between adjacent features) requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, X-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the photomask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through the photomask causes a chemical transformation in the exposed areas of the coating thereby making the image area either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. Using light having smaller wavelengths to selectively expose photoresists prior to development increases the resultant resolution. This is because precise control over the exposure area is increased as the wavelength of light decreases. As a result, there is trend toward the use of photoresists that are patterned using light having a relatively short wavelength. However, there are several concerns associated with using shorter wavelengths of light. As the wavelength of light decreases, the penetration depth of that light into a given photoresist generally decreases. This is a problem when most photoresists are coated on a semiconductor substrate at a thickness between 10,000 .ANG. and 20,000 .ANG..
Simply applying a thinner coating of a photoresist does not enable adequate and/or reliable use of the photoresist. This is because coated photoresists typically contain pinhole defects. Pinhole defects inhibit crisp pattern formation and critical dimensional control in developed photoresists. When a photoresist has a thickness of 10,000 .ANG. or higher, pinhole defects are not a concern since the pinholes are relatively small in relation to the photoresist thickness. As the thickness of a photoresist decreases, the deleterious effects, of pinhole defects increases. This is shown in FIG. 1. FIG. 1 illustrates a substrate 10 with a thin photoresist 12 having a thickness of about 5,000 .ANG.. The thin photoresist 12 has a number of relatively large pinholes 14; that is, relatively large compared to the photoresist thickness. Minimizing the deleterious effects of pinhole defects or the occurrence of pinhole defects in relatively thin photoresists is therefore desired.